1. Field of the Invention
This invention pertains generally to ion mobility spectrometers, and more particularly to photoemissive ion mobility spectrometers.
2. Description of Related Art
Ion mobility spectrometry (IMS) using a radioactive ionization source has become the standard method for the detection of trace quantities of explosives. Detection of chlorinated solvents, such as trichloroethylene and tetrachloroethylene, in the air and soil remains important for providing a safe environment. The advantages of IMS are its sensitivity in the ppb or pg range, its continuous real-time monitoring capability, its relatively low cost, and its portability due to instrumental simplicity.
Currently available commercial IMS instruments use radioactive sources that have significant deficiencies. Successful examples of nonradioactive sources for IMS using thermionic ionization, photoionization, laser ionization, corona discharge ionization, and electrospray ionization have been reported for a variety of applications.
Traditional IMS instruments employ β-particles (high-energy electrons) emitted from a 63Ni radioactive source to ionize nitrogen, a dominant component of carrier gases. Before analyte is ionized and able to be detected, a very complex sequence of ion-molecule reactions and energy-loss collisions occurs that produces both positive and negative reactant ions including low-energy electrons. When trace analyte vapor exists in the carrier gas, ion-molecule reactions between reactant ions and analyte neutrals generate product ions characteristic of the analyte. In the drift region, the ions assume characteristic drift velocities in the applied electric field such that at the end of the drift region individual ion products arrive at a collector electrode in swarms. The swarms appear as peaks in an ion mobility spectrum, which is simply a plot of collector current as a function of time. Drift times of the peaks permit identification of the ion products.
The use of photoemitted electrons from thin metallic films exposed to ultraviolet light in an electron-capture detector (ECD), eliminating the need for the radioactive 63Ni emitter, has been previously reported (Simmonds, P. G. Journal of Chromatography 1987, 399, 149-64). In air, the photoemitted electrons start with a maximum energy of only 0.4 eV and very rapidly acquire a mean electronic energy at 293 K equal or close to 3/2 kT (0.037 eV). Consequently, no positive ions or energized radicals are produced. Elimination of the positive ions prevents a large electron-positive ion recombination in the reaction region. The chemical processes within the detection cell are simplified considerably if positive ions are absent thus making interpretation of the measured signal more straightforward.
Begley et al. demonstrated the detection of trace analyte vapor in air using an ion mobility spectrometer fitted with a photoemissive ionization source (PE-IMS). They (Begley et al., Journal of Chromatography 1991, 588, 239-49) found that where oxygen is present, the ion mobility resolution is effectively unchanged at oxygen concentrations in excess of 6% and decreases as the oxygen content is decreased due to free electrons traveling further distances before being attached to oxygen. The Begley device was demonstrated for acetylacetone, benzoylacetone and benzoquinine. The Begley et al. device, however, used normal incidence illumination of the ionization source resulting in transmission of U.V. light into the ionization chamber, which lowers the free electron count and sensitivity and makes photochemical reactions in the ionization chamber likely.
Because detection of electronegative compounds such as chlorocarbons and nitro-organics require only low-energy electron attachment, electron photoemission (PE) is an advantageous alternative ionization source. The low-energy electrons generated are unable to ionize gas molecules by impact, which would produce both positive and negative ions. Instead, electronegative components of the carrier gas, including analyte if present, attach electrons with no loss due to recombination of positive and negative ions. Another advantage is that the number of electrons generated increases with the intensity of the incident light, permitting the number of electrons to be tailored to a particular analysis. Due in part to these advantages, photoemissive sources have appeared frequently throughout the past century in experimentation requiring low-energy electrons, including both the ECD and extensive implementation in instruments for physical studies of electron interactions with gas molecules.
Therefore an object of the present invention is a photoemissive ion mobility spectrometer with improved sensitivity for detection for compounds such as chlorocarbons and nitro-organics.